This invention relates to a rate-of-flow meter, particularly for diagnostic spirometry, containing a measuring stud provided with pressure taps and an adjustable displacement body.
In diagnostic spirometry such rate-of-flow meters are known as pneumotachographs. Well known is the Fleisch pneumotachograph in which a theoretically linear relationship between the measured pressure and the volumetric rate of flow is achieved by maintaining laminar flow through a plurality of parallel vanes of small cross section to create the desired pressure differential between the pressure taps. Objections to this rather complex and expensive instrument are the difficulty of keeping it clean and of disinfecting it, its relatively great weight and the rather poor signal strength that is available for measurement. This signal is provided exclusively by the pressure drop which arises during flow through the large number of parallel passages between the vanes. Any increase in signal strength therefore involves a corresponding increase in flow resistance. In a respiratory examination of the functioning of the lung it is desirable that this resistance should be a minimum. Other difficulties in practice arise because flow through the several channels may not be uniform and particularly because this may differ when the breath is inhaled and when it is expelled. Moreover, the measured pressure differential in laminar flow also depends on the viscosity of the medium. Finally, obstructions for instance by expectorated phlegm when coughing may considerably affect the results of the measurement. Other drawbacks are the relatively large unused capacity which causes errors when examining gas exchange, and the high demands on accuracy which the very small measuring signal makes on the efficiency of the electrical indicating system.
A more recent pneumatometer (German Specification published as-filed under No. 2,044,101) which is based on the same physical principle as the Fleisch pneumotachograph also avails itself of the pressure drop in laminar flow for making the measurement.
Another proposal relating to the design of a pneumatometer (German Specification published as-filed under No. 1,963,349) for generating the measured pressure avails itself of a flat flow resistor which completely fills the cross section of the flow tube. Such a flow resistor may be for instance a fine mesh. However, such resistors have the major disadvantage that the pressure drop they cause, being rendered dimensionally indeterminate by reference to a static pressure, depends in a complicated way upon the Reynold's number, particularly in the case of screens and nets. Consequently in the description of pneumatometers based on this working principle (German Specifications published as-filed under Nos. 1,963,349 and 2,000,800) there is always the qualification that the measuring accuracy that can be achieved is not very high.
Another rate-of-flow meter used in spirometry comprises two pitot tubes concentrically inserted in a breathing tube and operating in contrary directions, the static heads in each being applied to a differential pressure gauge (German Specification published as-filed under No. 1,153,486). In this arrangement the pitot tube which faces the direction of flow provides the total head whereas that pointing in the direction of flow approximately represents the static head. In order of magnitude the pressure difference therefore corresponds to the static pressure head. A high static head which would be desirable in the interests of improved measuring accuracy can be achieved only by a fairly considerable constriction of the cross section of the breathing tube. Such a constriction naturally represents an increase in resistance to flow. Consequently the static heads in a breathing tube cannot be very high. A drawback which appears particularly when measuring low rates of flow is the dependence of the pattern of flow through the two pitot tubes upon the Reynold's number. Owing to this effect the relationship between measured pressure and volume rate of flow may considerably deviate from the expected parabolic relationship.